Remove 'True' flash message when session expires.

This is because devise uses flash to store logic, instead of only
information that has to be given to the user. The 'timedout' key has
been removed from the flash hash. This should be reverted when devise
changes this.

app/views/partials/_flash.html.haml

@@ -1,3 +1,3 @@
-- flash.each do |key, value|
+- flash.delete(:timedout).each do |key, value|
   .pure-alert{ class: "pure-alert-#{key}" }
     = value